Canberra 2015A, User Manual

Page: 42 / 56

Window ( E) Threshold Circuits
Here the raw +24 volts is used to generate the WINDOW (E) reference voltage. Tran-
sistor Q27 is a current source generating a current through the WINDOW (E) control
(RV6). Resistors R118, pot RV8, and transistor Q28 are used to bias the current
source. Transistor Q28 also provides a temperature compensation for Q27. The ULD
Cal pot (RV8) is adjusted for 4 volts across RV6. Pin2 of the WINDOW (
∆
E) potenti-
ometer is connected to the OUTPUT of the LOWER LEVEL (E) discriminator buffer
amplifier, A15; thus the voltage developed across RV6 is summed with the LOWER
LEVEL reference voltage. Some fraction of this additive voltage is picked off using
RV6, buffered by A10, and connected to A9 pin 2 providing the WINDOW (
∆
E) ref-
erence voltage. Resistors R122, R123, and diode D20 form a 5 volt clamp preventing
the buffer Amp OUTPUT, A10, from exc5 volts. RV9 nulls out the window
buffer amp (A10) OUTPUT offset.
Fast Discriminator Reference Voltage
Op-amp A16 generates a reference voltage, having a nonlinear characteristic for fast
discriminator (A14). R109 through R1 13 sets up a bias voltage of approximately –80
mV. When the LOWER LEVEL (E) discriminator is less than 100 mV, diode D19 be-
comes reversed biased and the junction of R110 and R1 09 is held at approximately
–80 mV. This is attenuated to approximately –20 mV by R109 and R1 13 and repre-
sents +90 mV when referenced to the AMP OUTPUT signal. Diode D18 clamps the
OUTPUT of A16 pin 6 to +600 mV, preventing A16 from saturating. As the LOWER
LEVEL (E) control is increased above 100 mV, diode D19 begins to conduct increas-
ing the fast discriminator reference voltage. Further increases in the LOWER LEVEL
(E) control cause further increases in the fast discriminator reference voltage. This re-
lationship continues until the junction of R110 and R109 reaches approximately –600
mV at which point diode D17 becomes forward biased, clamping the amplifier
OUTPUT. Op-amp A16 will no longer track the LLD control, the fast discriminator
reference voltage has reached its upper limit, –600 mV, at the junction of R109 and
R110 is approximately 158 mV between pins 2 and 3 of the fast discriminator and rep-
resents approximately 700 mV when referenced to the AMP OUTPUT. The nonlinear
fast discriminator references voltage characteristic allows the LOWER LEVEL (E)
control to adjust the fast discriminator just above the noise region while maintaining
optimum timing performance.
5 Volt Power Supply
Resistors R170 and R39 attenuate the +12 V power supply to 5 volts. Diode D24 com-
pensates for temperature related V
be
variations of transistor Q26. Transistor Q26 is an
emitter follower and supplies the nec5 volt current. Resistor R171 acts as a
current limiter protecting the 5 volt supply in the event of a short circuit.
36
Circuit Description